Electroplating of iron and coating substrates with an iron-aluminum coating

ABSTRACT

A method of electroplating comprising employing an electroplating bath containing an electrolyte selected from the group comprising ferrous chloride and ferrous sulphate and in which is dissolved an amount of an additive selected from the group of citrates and citric acid which inhibits the formation in the bath of insoluble ferric compounds, iron being periodically added to the bath at a rate sufficient to reduce substantially all the ferric citrate formed therein by aeration.

Elaite Sites atet [191 Topham Mar. 18, 1975 1 1 ELECTROPLATING OF IRON AND 3,547,787 12/1970 Parsons 204/48 X CO S S S WITH AN 3,755,090 8/1973 Jackson et al. 204/34 IRON-ALUMINUM COATING [75] Inventor: Frederick William Topham,

Deeside, Wales [73] Assignee: British Steel Corporation, London,

England [22] Filed: June 4, 1973 [21] App]. No.: 366,760

[30] Foreign Application Priority Data June 20, 1972 United Kingdom 28775/72 [52] U.S. Cl 204/28, 204/37 R, 204/38 S, 204/48 [51] int. Cl. C23b 5/04, C23b 5/52, C23b 5/58 [58] Field of Search 204/48, 34, 38 S, 28, 37 R [56] References Cited UNITED STATES PATENTS 2,809,156 10/1957 Safranek 204/48 X OTHER PUBLICATIONS Frederick A. Lowenheim, Modern Electroplating, pp. 229 & 235, (1968).

R. M. Schaffer et a1., Trans. of the Electrochemical Soc., Vol. 84, pp. 319328, (1943).

Primary Examiner-G. L. Kaplan Attorney, Agent, or Firm-Bacon & Thomas [57] ABSTRACT A method of electroplating comprising employing an electroplating bath containing an electrolyte selected from the group comprising ferrous chloride and ferrous sulphate and in which is dissolved an amount of an additive selected from the group of citrates and citric acid which inhibits the formation in the bath of insoluble ferric compounds, iron being periodically added to the bath at a rate sufficient to reduce substantially all the ferric citrate formed therein by aeration.

13 Claims, No Drawings ELECTROPLATING OF IRON AND COATING SUBSTRATES WITH AN IRON-ALUMINUM COATING This invention concerns a method of electroplating, e.g. a method of electroplating a ferrous substrate with a coating of metallic iron.

When a conventional ferrous chloride plating bath is employed to provide a ferrous substrate with an electrolytic coating of metallic iron, the bath suffers a gradual oxidation during operation, such oxidation resulting from the reaction of the ferrous ions with atmospheric oxygen to produce ferric chloride. Some of this ferric chloride will be reduced to the ferrous state at the cathode during plating, but the majority of the ferric chloride is hydrolysed to form ferric hydroxide. Thus the conventional ferrous chloride bath quickly becomes contaminated with insoluble ferris compounds.

According to the present invention, there is provided a method of electroplating comprising employing an electroplating bath containing a ferrous chloride or ferrous sulphate electrolyte in which is dissolved an amount ofa citrate or citric acid additive which inhibits the formation in the bath of insoluble ferric compounds, iron being periodically added to the bath at a rate sufficient to reduce substantially all the ferric citrate formed therein by aeration.

The electrolyte is preferably a ferrous chloride electrolyte, although a ferrous sulphate electrolyte may alternatively be used.

The additive may for example be trisodium citrate.

The citrate ion concentration is the said electrolyte may be in the range of 2 to 32 grams per litre.

The method may be employed to provide a ferrous substrate with a coating of metallic iron.

Thus the substrate may be an iron alloy substrate such as mild steel, and may be passed through the bath during plating.

The pH of the bath may be in the range 1.0 to 5.0. Thus the pH of the bath may be in the range 2.8 to 5.0, e.g. between 4.0 and 4.5.

The electrolyte preferably contains not more than grams per litre of soluble ferric compounds.

The electrolyte preferably contains from 50 to 150 grams per litre of ferrous iron.

The use of the said additive in the electrolyte results in the formation of a ferric citrate complex by reaction with the ferric ions present in the solution. This ferric citrate complex resists hydrolysis.

The said ferric citrate complex will react with iron or steel objects in contact with the electrolyte solution to produce ferrous chloride.

Citric acid is a weak acid. The citric ions will therefore combine with some of the hydrogen ions in solution to produce un-dissociated citric acid, thus lowering the hydrogen ion concentration as indicated by the equation 3H +Cit. H Cit.

It is possible therefore to operate the bath successfully at pH values in excess of 4, as indicated above, hydrochloric acid being added at regular intervals to maintain the desired pH value.

The temperature of the bath during the said plating is preferably in the range 90C.

The thickness of the electrolytic coating of iron may be in the range 1.0 to 3.0 microns.

A current density in the range 150 to 500 amps per square foot may be employed.

The iron which is periodically added to the bath at a rate sufficient to reduce substantially all the ferric citrate formed therein by aeration may be in the form of scrap iron. Thus the total ferric ion concentration can be controlled at an acceptable level.

The bath temperature, and to a lesser extent, the current density have a marked effect on the visual appearance of the electrolytic coating of iron. For example, at a current density of 250 amps per square foot (26.6A/sq.dm), the deposit obtained at 10 to 30C has a matt metallic appearance. As the temperature of the electrolyte is raised, the appearance of the deposits at this current density darkens to a matt dark grey at 60C. Thereafter it lightens slightly.

For a given temperature, the deposit becomes lighter and more metallic in appearance the higher the current density used. The iron coated ferrous substrate obtained by the use of the bath of the present invention is both hard and brittle when compared to a similar product obtained from a ferrous chloride bath when no said additive is present, but the product is particularly suitable, whether the electrolytic coating of metallic iron is in the matt metallic or matt dark grey form, for use in the process described in our British Pat. No. 1,203,473.

Thus after the substrate has been provided with said coating of iron, the said coating of iron is preferably wetted with an aqueous alkali metal silicate solution, the wetted coating of iron is coated with a layer of aluminium powder which forms an alloy with the substrate when heat treated at a temperature within the range 500-650C, the iron coating and the aluminium powder layer is compacted to the surface of the substrate at a temperature at which sintering will not occur during the compaction, and the compacted substrate is heat treated at a temperature within the range 500-650C. so that sintering and bonding of the aluminium particles to each other and to the substrate occurs.

The said heat treatment may for example be in the range 502 to 600C, and preferably in the range 502 to 525C.

Thus a notable property of the electrolytic coating of iron produced by the method of the present invention is that it inhibits the formation of the said alloy. That is to say, one of the outstanding properties of aluminium coated steel is its oxidation resistance at elevated temperatures. At temperatures below about 500C the product remains bright and lustrous combining the well-known oxidation resistance of pure aluminium with the strength of steel. Above about 650C, the aluminium coating forms an alloy with the steel substrate by diffusion, yielding a dark grey surface which again has oxidation resisting properties. Between these two temperatures, however, diffusion occurs slowly and the aluminium coating is separated from the steel substrate by an intermediate layer of alloy, the thickness of which depends upon the temperature and the length of time for which the aluminium coated steel is subjected to this temperature. Whilst the presence of this intermediate layer is not normally detrimental during service, it is undesirable in the product prior to fabrication because its brittle nature causes the aluminium coating to crack and even delaminate in the forming operation. Moreover, if the product is used for long periods at a temperature in the range 500 to 650C, the aluminium coating can blister and in some cases become detached from the steel substrate. This is because in the initial stages of alloying, the adhesion between the aluminium coating and the thin alloy layer is of a very low order and thermal cycling can cause micro-separation leading to delamination.

It is believed, however, that the provision of the electroplated layer of iron retards the nucleation of an iron- /aluminium layer by raising the temperature at which little or no nucleation occurs. This temperature is, it is believed, raised by reason of the higher-purity of the iron layer compared to the ferrous substrate. The steel is of course impure iron in the sense that it has other constituents alloyed with the iron.

When nucleation commences, the growth of the alloy from given nucleates is very rapid due to the fast rate of reaction at the raised nucleation temperature. As a result, the risk of blistering and delamination is reduced. Thus the nucleation appears to be controlled such that the alloy grows in a toothy pattern into the aluminium layer, thereby promoting adhesion of the layers to each other.

Thus the plating effected by the method of the present invention enables very good adhesion to be achieved between the coating and the substrate. In fact, samples of aluminium coated steel produced as indicated above have been maintained at a temperature of 620C for 3 hours without alloy formation at the interface between the aluminium and iron layers.

The present invention enables a wider range of pH values to be selected than was previously possible, the citrate ions acting as a buffer and enabling the bath to be operated at high pH values without the need of alkali additions. The addition of the citric acid or citrate also has a considerable effect on the nature of the iron coating produced, resulting in increased aluminium coating adhesion and in greater resistance to the formation of the aluminium/iron alloy in the temperature range 500 to 650C. Additionally, the invention enables a satisfactory iron coating to be produced at bath temperatures below 50C.

The invention is illustrated in the following Examples.

EXAMPLE 1 A mild steel strip to be coated is wound from a roll thereof and passes at a speed of up to 300 ft per minute through two degreasing baths where the strip is degreased by the use of a hot alkaline degreasing liquid. The strip is then rinsed in cold water after which it is lightly etched in dilute hydrochloric acid so that all the surface area of the strip is cleaned by having any metallic oxide or other contamination removed therefrom. The strip is then rinsed in cold water, and passes to a plating bath immediately before which it is thoroughly rinsed either by the electrolyte itself, or, preferably, by an acid dip, e.g. an HCl solution of pH 1.0.

The plating bath employed has a ferrous chloride electrolyte of the following composition:

Fe 1.0 g/l Trisodium citrate 5.8 g/l The pH value of the bath is 4.2, and the temperature of the electrolyte is 50C.

The thickness of the iron deposited can vary between 1.0 and 3.0 microns but an optimum value appears to be 2.0 microns thickness.

The plated strip is then subjected to a cold water rinse, an acid rinse (e.g. in an HCl solution of pH 1.0), and a further cold water rinse, followed by a hot water rinse, after which the plated strip is dried.

The plated strip is then sprayed with a 0.3% solution of sodium or potassium silicate at the range of 2.5 cc/sq. ft. After this a layer of aluminium particles, e.g. of a particle size 300 mesh/dust, is applied in an electrostatically charged condition to each of the opposite sides of the moving strip on top of the layer of iron thereon at a rate of say 10 grams per square foot, although this amount may be varied considerably if desired. The strip then passes through two drying stations, each incorporating a high frequency heater.

The strip is then passed between the rolls of a rolling mill to compact the aluminium powder thereto. lf desired, a solution of sodium carboxy-methyl-cellulose may be applied before compaction so as to reduce the extent to which the said rolls might otherwise dislodge that aluminium powder. Thus there is a reduced tendency for banks of dislodged powder to build up immediately ahead of the rolls and thereby prevent the strip being moved at high speeds.

The compacted strip is then coiled for subsequent heat treatment, the latter consisting in heating the coil in air for 15 to 20 hours at 502 to 600C, and prefera bly in the range of 502 to 525C. The compaction arising from the use of the said rolls merely produces a mechanical bond between the particles ofaluminium powder and the electroplated iron layer so that the particles of aluminium powder are merely mechanically secured in position without being sintered to each other and to the steel substrate. However, the said heat treatment causes the aluminium particles to be sintered and bonded to each other and to the steel substrate.

In the Examples 2 to 5 given below, the process is the same as in Example 1, but the compositions, pH values, and temperatures of the baths employed are somewhat different.

EXAMPLE 2 Fe 71.4 g/l Fe below 1.0 g/l Trisodium citrate 5.7 g/l pH value 2.8

Electrolyte Temperature 20C.

EXAMPLE 3 Fe below 1.0 g/l Trisodium citrate 20 g/l pH value 3.2

Electrolyte Temperature C EXAMPLE 4 Fe 103.7 g/l Fe below 1.0 g/l Trisodium citrate 10 g/l pH value 3.0

Electrolyte Temperature 45C.

EXAMPLE 5 Fe 65.0 g/l Trisodium Citrate 9 g/l pH value 4.5

Electrolyte Temperature C.

The invention also comprises a ferrous substrate which has been provided with a metallic coating by the method set forth above.

I claim:

l. A method of forming a coating on a substrate comprising an electroplating step utilizing a cathodic current density in the range of approximately 150-500 amps per square foot while employing an acidic, aqueous electroplating bath containing an additive which is selected from the group consisting of citrates and citric acid, the citrate ion concentration in the bath being in the range of approximately 2-32 grams per litre and the rest of the bath being made up of an electrolyte which is selected from the group consisting of ferrous chloride and ferrous sulphate and which contains approximately 50150 grams per litre of ferrous iron, with iron being periodically added to the bath at a rate sufficient to reduce substantially all the ferric citrate formed therein by aeration.

2. A method as claimed in claim 1 in which the method is employed to provide a ferrous substrate with a coating of metallic iron.

3. A coating method as claimed in claim 2 further including the steps of:

A. wetting the iron coating with an aqueous alkali metal silicate solution,

B. coating the wetted iron coating with a layer of aluminum powder which forms an alloy with the substrate when heat treated at a temperature within the range of 500650C,

C. compacting the aluminum powder layer and the iron coating to the surface of the substrate at a temperature at which sintering will not occur during the compaction, and

D. heat-treating the compacted substrate at a temperature within the range of 500650C to sinter and bond the aluminum particles to each other and to the substrate.

4. A method as claimed in claim 3 in which the heat treaatment is conducted at a temperature in the range 502-525C.

5. A method as claimed in claim 2 in which the ferrous substrate is passed through the bath during platmg.

6. A method as claimed in claim 1 in which the pH of the electrolyte is in the range 1.0 to 5.0.

7. A method as claimed in claim 6 in which the pH of the bath is in the range 2.8 to 5.0.

8. A method as claimed in claim 7 in which the pH of the bath is between 4.0 and 4.5.

9. A method as claimed in claim 1 in which hydrochloric acid is periodically added to the bath to maintain the pH of the latter at a desired value.

10. A method as claimed in claim 9 in which the thickness of the coating of iron is in the range 1.0 to 3.0 microns.

11. A method as claimed in claim 1 in which the additive is trisodium citrate.

12. A method as claimed in claim 1 in which the electrolyte contains not more than 5 grams per litre of soluble ferric compounds.

13. A method as claimed in claim 1 in which the temperature of the bath during the said plating is in the range 20 to C. 

1. A METHOD OF FORMING A COATING ON A SUBSTRATE COMPRISING AN ELECTROPLATING STEP UTILIZING A CATHODIC CURRENT DENSITY IN THE RANGE OF APPROXIMATELY 150-500 AMPS PER SQUARE FOOT WHILE EMPLOYING AN ACIDIC, AQUEOUS ELECTROPLATING BATH CONTAINING AN ADDITIVE WHICH IS SELECTED FROM THE GROUP CONSISTING OF CITRATES AND CITRIC ACID, THE CITRATE ION CONCENTRATION IN THE BATH BEING IN THE RANGE OF APPROXIMATELY 2-32 GRAMS PER LITRE AND THE REST OF THE BATH BEING MADE UP OF AN ELECTROLYTE WHICH IS SELECTED FROM THE GROUP CONSISTING OF FERROSU CHLORIDE AND FERROUS SULPHATE AND WHICH CONTAINS APPROXIMATELY 50-150 GRAMS PER LITRE OF FERROUS IRON, WITH IRON BEING PERIODICALLY ADDED TO THE BATH AT A RATE SUFFICIENT TO REDUCE SUBSTANTIALLY ALL THE FERRIC CITRATE FORMED THEREIN BY AERATION.
 2. A METHOD AS CLAIMED IN CLAIM 1 IN WHICH THE METHOD IS EMPLOYED OF PROVIDE A FERROUS SUBSTRATE WITH A COATING OF METALLIC IRON.
 3. A COATING METHOD AS CLAIMED IN CLAIM 2 FURTHER INCLUDING THE STEPS OF: A. WETTING THE IRON COATING WITH AN AQUEOUS ALKALI METAL SILICATE SOLUTION, B. COATING THE WETTED IRON COATING WITH A LAYER OF ALUMINUM POWDER WHCH FORMS AN ALLOY WITH THE SUBSTRATE WHEN HEAT TREATED AT A TEMPERATURE WITHIN THE RANGE OF 500*-650*C, C. COMPACTING THE ALUMINUM POWDER LAYER AND THE IRON COATING TO THE SURFACE OF THE SUBSTRATE AT A TEMPERATURE AT WHICH SINTERING WILL NOT OCCUR DURING THE COMPACTION, AND D. HEAT-TREATING THE COMPACTED SUBSTRATE AT A TEMPERATURE WITHIN THE RANGE OF 500*-650*C TO SINTER AND BOND THE ALUMINUM PARTICLE TO EACH OTHER AND TO THE SUBSTRATE.
 4. A method as claimed in claim 3 in which the heat treaatment is conducted at a temperature in the range 502*-525*C.
 5. A METHOD AS CLAIMED IN CLAIM 2 IN WHICH THE FERROUS SUBSTRATE IS PASSED THROUGH THE BATH DURING PLATING.
 6. A method as claimed in claim 1 in which the pH of the electrolyte is in the range 1.0 to 5.0.
 7. A method as claimed in claim 6 in which the pH of the bath is in the range 2.8 to 5.0.
 8. A method as claimed in claim 7 in which the pH of the bath is between 4.0 and 4.5.
 9. A method as claimed in claim 1 in which hydrochloric acid is periodically added to the bath to maintain the pH of the latter at a desired value.
 10. A method as claimed in claim 9 in which the thickness of the coating of iron is in the range 1.0 to 3.0 microns.
 11. A method as claimed in claim 1 in which the additive is trisodium citrate.
 12. A method as claimed in claim 1 in which the electrolyte contains not more than 5 grams per litre of soluble ferric compounds.
 13. A method as claimed in claim 1 in which the temperature of the bath during the said plating is in the range 20* to 90*C. 